1. Field of the Invention
The invention is related to methods and apparatuses for treating infections in living tissue, such as may occur in wounds. Particularly, the invention is related to methods and apparatuses for promoting wound healing by utilizing low temperature, atmospheric pressure plasmas. The invention also is related to a new use of atmospheric pressure plasmas for living tissue treatment.
2. Description of the Related Art
Wounds are a major source of infection. Once infected, wounds heal slowly, and if not treated aggressively can lead to amputation and even death. Recently, it was reported that in Pennsylvania alone, over 11,600 hospital patients became infected, leading to an additional 1,500 deaths and $2 billion in hospital charges (see e.g., USA Today, Jul. 13, 2005, page 3B). There are many different types of wounds, including surgical wounds, burns, ulcers, compound fractures, bedsores, and tissue necroses. Chronic skin ulcers are common in patients with reduced blood circulation due to diabetes mellitus, atherosclerosis, arteriosclerosis obliterans, Buerger's disease, and excessive radiation therapy. Skin ulcers are highly susceptible to infection. Infected wounds can cause secondary infections in other parts of the body, such as in the bone or bone marrow in osteomyelitis.
Soldiers receive complicated invasive wounds from gunshots, incendiary devices, improvised explosive devices (IEDs), land mines, and many other types of ordinance. Infection can set in at any time as the soldier is moved from the battlefield to the field hospital to recovery facilities. Once infected, battlefield wounds heal slowly, and treatment is a painful, prolonged process.
A variety of bacteria can invade wounds and prevent them from healing. These bacteria may be classified as gram-positive or gram-negative, and as obligate aerobes, facultative anaerobes, microaerophilic, or obligate anaerobes (see Gladwin and Trattler, in “Clinical Microbiology Made Ridiculously Simple,” MedMaster, Inc., Miami, Fla., 2004). Among these, the microorganisms most likely to infect tissue, including skin and bone, are Streptococcus pyogenes, Staphylococcus aureus, Clostridium perfringens, and Pseudomonas aeruginosa. Skin infections from Streptococcus and Staphylococcus can result in impetigo, cellulitis, abscesses, furuncles, and carbuncles. Streptococcus can enter deep wounds and spread rapidly through the fascia between the skin tissue and muscle, causing Necrotizing Fasciitis, a condition that results in skin death and is often fatal. Clostridium perfringens, otherwise known as Gas Gangrene, invades deep wounds with dead tissue, where an anaerobic environment exists. This infection used to be common among soldiers wounded in battle.
Pseudomonas aeruginosa is common in hospitals, infecting sick patients with weak immune systems. This bacterium is a gram-negative, obligate aerobe. It is resistant to nearly all antibiotics. Diabetic patients have an increased risk of developing foot ulcers colonized with Pseudomonas, and it can penetrate into the bone causing osteomyelitis. This bacterium is very common in burned tissue, infecting over ⅕th of all burn victims (see Kluytmans, “Surgical infections including burns,” in Prevention and Control of Nosocomial Infections, Wenzel, ed., Williams and Wilkins, Baltimore, Md., 1997). If not eradicated, this infection eventually leads to fatal sepsis. Another microorganism that can similarly infect burns is Pseudomonas cepacia. 
When bacteria invade wounds, they can quickly develop into a biofilm (see Wolcott, “Biofilm Based Wound Care,” Southwest Regional Wound Care Center, Lubbock, Tex., 2005). A biofilm is a multicellular organism with strong defenses that makes eradication difficult, and is one of the main reasons chronic wounds do not heal (see Costerton, Stewart, and Greenberg, “Bacterial Biofilms: A common cause of persistent infections,” Science, Vol. 284, p. 1318, 1999). Pseudomonas aeruginosa is an excellent example of biofilm forming bacterium, and is why this infection is so tenacious in ulcers and burned tissue. In order to treat, biofilm-based infections in wounds, a multi-pronged approach is needed that includes mechanical debridement (cutting and scraping), antibiotics, and topical antimicrobial agents. The former technique can be painful for the patient, and the wound can become infected again shortly after treatment. Moreover, bacteria are constantly evolving and in some cases are showing increased resistance to antibiotics and antimicrobial agents. Therefore, it is clear that new treatments are necessary to kill bacteria that invade wounds, and to further assist the body in the healing process.
Over the past several years, it has been shown that bacteria can be destroyed by exposure to atmospheric pressure plasmas (see for example, Laroussi, IEEE Transactions on Plasma Science, Vol. 30, p. 1409, 2002; Kelly-Wintenberg, Montie, et al., Journal of Industrial Microbiology and Biotechnology, Vol. 20, p. 69, 1998; and Herrmann, Henins, et al., Physics of Plasmas Vol. 6, p. 2284, 1999). The gas in these plasmas is weakly ionized so that the temperature remains low, i.e., near ambient conditions. In these studies, atmospheric pressure plasmas have been used to kill microorganisms dispersed on gel or glass media. Researchers have found that the effectiveness of the technique varies widely depending on the culture medium and how it is prepared, the specific plasma device used, and the method of plasma generation. Note that none of these studies focused on bacteria commonly found in wounds. In one case, the treatment of biofilms with atmospheric pressure plasma was examined (see Joaquin, Abramzon, and Brelles-Mariño “Gas Discharge Plasmas as a Novel Approach to Destroy Bacterial Biofilms,” Applications in Environmental Microbiology, submitted in 2005). The researchers found that exposure of Rhizobium gallicum biofilm to the plasma for 5.0 minutes killed from 96.9 to 99.9% of the colony forming units.
A group of Russian researchers have shown that treatment of purulent wounds in rats with gaseous nitric oxide reduced healing time by 32% compared to the control group (see Shekhter, Serezhenkov, et al., “Beneficial effect of gaseous nitric oxide on the healing of skin wounds,” Nitric Oxide, Vol. 12, p. 210, 2005). The nitric oxide was dispensed onto the wound at levels near 1000 ppm, and was produced using an air plasma device. These authors stated that they have treated over 10,000 patients with a wide variety of skin wounds, finding that gaseous nitric oxide promotes wound healing in most cases. Nevertheless, NO treatment of wounds has not proven to be effective in studies conducted in the United States, and in some cases, such as in burn injury, it could be detrimental to the patient (see Soejima, Traber, et al., “Role of Nitric Oxide in Vascular Permeability after Combined Burns and Smoke Inhalation Injury,” American Journal of Respiratory and Critical Care Medicine, Vol. 163, p. 745, 2001).
Due to the widespread infection of wounds, and their annual toll on human life, not to mention the enormous costs to the medical profession and society in general, there is a great need in the art to develop more effective methods of treating these injuries. These and other needs are met by the present invention as described in detail hereafter.